The invention relates generally to optical sensors for fluids. More particularly, the invention relates to optical sensors of the type used for fluid level detection and that can discriminate different fluid types.
Optical fluid point level sensors are well known. Such sensors commonly use a prism having a conical tip. The prism is transparent to a beam of light launched into the prism from a light source (such as an LED). The light travels through the prism towards a surface of the conical tip and impinges on the surface at a determinable angle of incidence. The prism is made of a material that has a refractive index such that there exists a critical angle of incidence at which all light is internally reflected to another surface and back to a photodetector, such as a phototransistor. Whether such internal reflection occurs depends on the refractive index of the fluid to which the conical tip is exposed and the angle of incidence. The critical angle is defined by the following equation: EQU .theta..sub.c =sin.sup.-1 (n.sub.2 /n.sub.1) Eq. 1
where n.sub.2 is the index of refraction of the fluid, and n.sub.1 is the index of refraction of the prism conical tip. Thus, for air, n.sub.2 =1.00 and for glass, n.sub.1 =1.50. Accordingly, for total internal reflection the critical angle with respect to air is about 42.degree.. By comparison, if the conical tip is exposed to water as the fluid, the refractive index of water is 1.33. Thus the critical angle for total internal reflection with respect to water is about 62.5.degree..
By forming the conical surfaces such that the light transmitted therethrough is incident at 45.degree., the light will undergo total internal reflection (hereinafter "TIR") when the conical tip is exposed to air (because 45.degree. is greater than the critical angle of 42.degree. for a glass/air interface), but will not undergo TIR when the conical tip is exposed to water (because 45.degree. is less than the critical angle of 62.5.degree. for a glass/water interface). By positioning a light detector to receive the light that is internally reflected, the prism can be used as a point level detector for the water level. The transmitted light that is not internally reflected is refracted into the fluid, as is well known.
Note that for TIR to occur, the refractive index of the conical tip is higher than the refractive index of all fluids which are to be detected (in this example, air and water).
Such a prismatic sensor can also be used to detect an aircraft fuel/air interface when the prism material is made of a higher refractive index such as 1.65, because the index of refraction for fuel is on the order of 1.4 to 1.5. Thus, it is known to use such sensors for fuel level detection by detecting the ullage/fuel interface at different levels in a fuel tank.
A significant problem in aircraft fuel tanks, particularly large commercial aircraft flying at high altitudes for extended periods of time, is the accumulation of free water at the bottom of the tank. This free water can adversely affect the performance of capacitance type fuel quantity sensors; although such erratic behavior can be used as a warning that water is accumulating in the tank. Typically, tank sumps are opened to drain the water from the tanks. Most aircraft also have scavenge pumps that are used to mix the water with the fuel and burn it off prior to buildup of any significant amount.
Free water is continually being generated in the fuel tanks. During ascent, the fuel cools and water is thrown out of solution. Further, during descent, moist air is sucked into the tanks and condensation occurs on the surface of the fuel and cooled structural members.
The fuel tanks on such aircraft can remain below 0.degree. C. for several hours after landing. As a result, the free water freezes and the scavenge pumps and sumps are ineffective. Even after refueling, the ice can remain for extended periods. Short layovers and improper fuel storage and fueling operations at remote locations can cause even more free water to be loaded on-board the aircraft.
Conventional optic fluid level sensors such as just described are ineffective in such circumstances because the prism/water interface does not cause TIR. Thus, water in the tank can be misinterpreted by such sensors as being fuel.
Optical sensors known heretofore also use three electrical conductors to access the sensor and couple the output to a fuel management system, which can add significantly to the weight of the aircraft.
Another significant drawback of known optic sensors is that the electronics housed in the each sensor tend to be very sensitive to operating temperatures and electromagnetic interference, thus requiring additional circuitry for temperature compensation and filtering.
The objectives exist, therefore, for an optical sensor for fuel level sensing that can discriminate between air, fuel and water. Such a sensor should also be able to detect ice as well as liquid water, and preferably should exhibit stable operation over a wide operating temperature range. Furthermore, such a sensor should use a minimal number of conductors for interfacing to control circuitry.